There is a need today for creating a low radar signature for different objects such as e.g. aircrafts, i.e. to design aircrafts having a low radar visibility. Significant progress has been achieved in a number of problem areas as e.g.:                Intake/exhaust        Cockpit/canopy        Hull or fuselage shape        Absorbers        Armamentbut there is often a problem with reducing the passive signature of the aircraft sensors such as antennas.        
A number of solutions have been proposed for antennas with a low radar signature or a low Radar Cross Section, RCS.
Antennas, as e.g. radar antennas in aircrafts, are often so-called array antennas i.e. antennas consisting of a number of antenna elements working together. In order to reduce the RCS of array antennas in a conductive hull WO 2006/091162 has proposed to frame the array with a thin and tapered resistive sheet. FIG. 1 shows a cross section of an antenna according to prior art. An antenna unit 101 with antenna radiators 102 and a dielectric cover 103 is mounted in a hull 104. A tapered resistive sheet 105 is applied as a frame on top of the antenna unit 101. By tapered is understood that the resistivity varies from “high resistivity” nearest to the antenna centre to “low resistivity” nearest to the conductive hull. This method is able to reduce the backscattering caused by discontinuities between antenna area and hull or fuselage substantially.
Although efficient this method has a problem with a relative high phase depth Δφ, see FIG. 1. Δφ, 106, is the difference in reflected phase from the hull and from the array region causing a large RCS.
The array is usually much thicker than the hull or fuselage, thus allocating an unnecessarily large volume in the aircraft.
Irrespective of array thickness, the integration causes a weakening of the hull or fuselage since the RF-active (RF=Radio Frequency), low loss materials in the array usually can not bear much mechanical stress. Extra, weight-consuming reinforcements must then be devised.
By applying the resistive layer at a significant height above the antenna radiators, a transmitted beam interferes with the resistive layer at moderate scan angels. This necessitates the introduction of a comparably large transition region (i.e. resistive sheet) which in turn makes the aperture in the hull or fuselage larger than necessary. FIG. 2 schematically illustrates the parameters affecting the width of the transition region. Antenna radiators 203 are located at a certain distance 204 from a hull 201. A first part 205 of the transition region is primarily depending on the operating frequency and shall have a width of N*λ. Normally it is sufficient with N=1-8. Higher N-values may however be necessary if very large RCS reductions are required. A second part of the transition region 207 is a function of the phase depth difference Δφ which exhibits some degree of proportionality to the distance 204. Finally a third part 209 of the transition region is a function of a scan angle α, also designated 211. A large scan angle means that the section 209 has to be wider which leads to the total transition region becoming larger.
This solution is most efficient for TE incidence (Transverse Electric polarization), but not for TM incidence (Transverse Magnetic polarization). The generally acknowledged solution to this problem is to introduce further (e.g. bulk-) absorbers inside the antenna near its edges. But again, this is associated with extra costs and increased width of the transition region. FIG. 3 explains the difference in handling of a TE wave, FIG. 3a, and TM wave, FIG. 3b, with a hull 301, an antenna 302 and a resistive sheet 303. An incident wave 305 propagates in the direction of the arrow. For a TE-wave the E-field is perpendicular to the plane of the paper illustrated with a circle and a dot. A TM-wave has the magnetic field in the same direction as the E-field in FIG. 3a. The E-field for the TM-wave is shown with an arrow 306. This means that the E-field for a TE-wave will have a direction along the resistive sheet and will be absorbed by the sheet. The TM-wave however will only have a small component in the direction along the resistive sheet and will therefore only be absorbed by the sheet to a small degree. The TM-wave will instead scatter at the antenna edge. A way to decrease this scattering is to include an absorbing material 307 at the end of the antenna. This however increases the width of the antenna and adds costs.
Gradually changing of the reflection coefficients, Γn, of the antenna radiators by introducing small changes of the element internal geometry that would give rise to a change of the reflection coefficient Γ has also been suggested as a means to reduce RCS. The proposition showed in FIG. 4 is aimed at changing the reflection coefficient Γ of dual-polarized antenna elements over the entire array surface, whilst keeping the transmit/receive losses as low as possible. Hence, reactive (capacitive/inductive) changes were considered, rather than resistive. FIG. 4 shows antenna radiators, in this case realized as waveguides, 401 with perturbations 402 and a hull 403. In the diagram of FIG. 4 a vertical axis 404 represents the reflection coefficient Γn, and a horizontal axis 405 represents the position of each antenna element n. The perturbations 402 are designed such that the reflection coefficient Γ is high close the outer edges of the antenna where the antenna meets the hull and low in the middle of the antenna thus creating a smooth transition from the high reflection coefficient of the hull to the low reflection coefficient of the antenna. This smooth transition reduces scattering and thus the RCS.
A drawback with this solution is that the reactive character of the perturbations implies that the signature reduction is only efficient over a limited bandwidth.
Another drawback is also that it is a very costly procedure to design a large number of individual antenna elements.
The method requires either that both polarisations be terminated and using dual polarized perturbations or, which is possible only in principle, that only one polarisation is terminated whilst introducing a single-polarized perturbation. The requirement that both polarizations be properly terminated is extra costly if the antenna function only requires one single polarization.
The phase depth 406 of the scattering is also a problem; it is not always possible to introduce the reactive perturbations in the plane where it would be optimal which is at the same level as a ground plane.
As mentioned above there are different types of backscattering causing a high RCS:                Edge scattering caused by discontinuities between antenna area and hull. This kind of scattering can be dealt with by applying a resistive layer as discussed above. The strength of the edge scattering is affected also by Δφ, i.e. the phase difference between the reflected signals from the hull and the antenna region. This scattering can to some extent be reduced by making the antenna as thin as possible.        Grating lobes scattering which will be discussed more in detail below.        
There is thus a need for an improved antenna solution integrated in the hull and having a low RCS at the same time as it is light weight and cost effective to produce.